The present invention relates generally to monitoring of spray painting systems and methods, and more particularly, to a spray paint monitoring system and method using Doppler radar.
Currently, automotive spray paint operations are required by emission laws and regulations to reduce the amount of volatile organic compounds that are emitted into the atmosphere. Conventional solvent borne spray paint processes presently used produce volatile organic compounds emissions of 4.0 lbs/gal, which just barely meets the federal and state (many but not all) pollution restrictions. It is anticipated that the restrictions will become much more stringent in the near future and that unless cleaner spray paint technology is developed expensive incineration facilities or other costly pollution controlling equipment will be required to control the pollution from paint operations.
A new spray painting process currently under development using super-critical carbon dioxide which is expected to lower the volatile organic compound emissions to 2.0 lbs/gal and which is expected meet the tighter requirements without requiring installation of pollution control equipment. However, to control the spraying process, a paint spray monitor is required. Specifically, in order to continually monitor and optimize the application of spray paint, it is necessary to provide for real time monitoring of the particle gun used in the spray painting system.
One conventional sensor that has been used in spray paint monitoring is a laser system that monitors the speed and size of individual paint particles. The laser approach uses two parallel helium-neon laser beams which were crossed as a result of being passed through a convex lens which had a focal length of 5 cm. An fringe pattern formed at the point where the laser beams intersected and the fringe spacing was approximately 5 .mu.m. The waist of the beam at the focal point was approximately 70 .mu.m, which was suitable for measuring particles in the 50-70 .mu.m range. As a paint particle boundary crossed the interference fringe pattern it produced a temporal intensity fluctuation which was governed by the velocity of the particle in order to measure such small particles a short working distance was therefore required and consequently the laser source and the light scattering detector needed to be inside the spray paint profile.
The laser approach was successful in measuring spray paint particle velocities. However, due to the small beam width many measurements throughout the paint spray profile were needed in order to determine the distribution. To test the equipment, a series of experiments were run to determine the particle velocities at a distance of approximately 12 in from the spray gun nozzle 32 and at 5 different points through the spray paint particle distribution (center, and 2 and 3 inches above and below center, relative to the gun nozzle position). Particle velocities were measured as a function of changing the pre-orifice and orifice size in the paint gun nozzle 32 as well as changes in the CO.sub.2 pressure applied to the gun. A trend to higher maximum mean particle velocity was observed when increasing the orifice size while maintaining the same pre-orifice size. The range of mean velocities was observed to change between 3 and 14 m/s over the course of all combinations of pre-orifice and orifice sizes used. It was also determined that for all combinations of pre-orifice and orifice sizes, a reduction in CO.sub.2 pressure approximately 27% from 1500 psi to 1100 psi resulted in approximately 15% reduction in the maximum mean paint particle velocity. In these cases velocity did seem to be a good indicator of changing paint gun parameters.
The disadvantage of the laser scattering approach is that it requires a short working distance (approximately 5 cm) which makes the measurement intrusive to the spray paint profile. Since the laser beam sampling volume is very small (approximately 10-13 m.sup.3) many measurements through the paint spray profile are necessary in order to determine the complete velocity distribution profile. This makes the measurements both time consuming and unsuitable for real time feedback monitoring. Consequently, the laser system is obtrusive (too close to the paint spray gun), too restricted in its monitoring area, and much too slow to accumulate gun data for real time process control.
Therefore, it would be desirable to have monitoring equipment and a closed loop control system and processing methods to improve spray paint quality control and provide cost savings resulting from increased transfer efficiency as a consequence of a higher charge transfer efficiency system on the paint gun. It would also be desirable to have monitoring equipment and a closed loop control system and processing methods that provide for a reduction in the emission of volatile organic compounds caused by spray paint operations.
Therefore, it is an objective of the present invention to provide for a spray paint monitoring system that may be employed with super-critical carbon dioxide spray painting systems. It is a further objective of the present invention to provide for a method that provides for real time monitoring of the performance of a particle gun used in a spray paint system by monitoring the paint spray produced thereby to optimize the application of spray paint. It is a further objective of the present invention to provide a system and method for closed loop control of a pressurized painting system.